1. Field of the Invention
The invention relates in general to the fabrication of a self-aligned silicide and more particularly to a method of manufacturing a cobalt silicide (CoSi.sub.2) layer.
2. Description of the Related Art
As the level of integration for MOS devices increases, resistance in the source/drain terminals of the MOS device gradually rises to a value comparable to the channel resistance of the MOS device. To ensure integrity at the shallow junction between metallic contacts and the MOS terminals, and for the downward adjustment of sheet resistance in the source/drain terminals, self-aligned silicide processes are now employed in the manufacturing of very large scale integrated (VLSI) circuits, especially for the manufacturing of semiconductor devices having a line width smaller than about 0.5 .mu.m.
Titanium is the metal material most frequently used for the self-aligned silicide. However, it is not easy to control the silicide step at the high temperature needed for the titanium salicide process. Moreover, the temperature used to transform the titanium metal into titanium silicide is very high. As the size of the titanium silicide is diminished, high temperature is needed to transform the phase completely, but high temperature bring about thermal unstability. In response to these problems cobalt silicide can be used to displace the titanium silicide in the future, because it has a low silicide temperature and low resistance. However, large amounts of silicon are consumed during the formation of the cobalt silicide, and it is not easy to control junction depth. Moreover, the formation of cobalt silicide is degraded by the native oxide formed on the surface of the silicon substrate.
FIGS. 1A to 1B are cross-sectional views showing a progression for manufacturing a cobalt silicide layer by using a cobalt/silicon structure according to a conventional method.
First, as shown in FIG. 1A, a silicon substrate 10 is provided. The silicon substrate 10 is dipped in a buffer oxide etchant (BOE) solution with a 50:1 concentration to remove the native oxide formed on the surface of the silicon substrate 10. Then, a layer of cobalt metal 12 is formed, for example, by using a sputtering method, over the silicon substrate 10. The sputtering method comprises, for example, a DC magnetically controlled sputtering method, at a base pressure of about 2.times.10.sup.-7 torr.
Next, referring to FIG. 1B, a thermal oxidation method is performed, for example, by using a rapid thermal process with nitrogen for about 30 sec, so that the cobalt layer 12 reacts with the silicon atoms on the surface of the substrate 10, forming a layer of cobalt silicide 14. Then, the unreacted and remaining cobalt metal are removed, for example, by using a wet etching method.
A conventional manufacturing method has several drawbacks including the roughness at the junction interface of the silicon substrate 10 and cobalt silicide layer 14, making it hard to control the junction depth. Additionally, the formation of the cobalt silicide is degraded by the native oxide formed on the surface of the substrate structure.
Another conventional manufacturing method comprises adding a layer of titanium to the interface between the cobalt layer and silicon substrate, thereby preventing the formation of the native oxide.
FIGS. 2A to 2B are cross-sectional views showing a progression for manufacturing a cobalt silicide layer by using a cobalt/titanium/silicon structure according to another conventional method.
First, as shown in FIG. 2A, a silicon substrate 20 is provided. Silicon substrate 20 is dipped in a buffer oxide etchant solution with a 50:1 concentration to remove the native oxide formed on the surface of the silicon substrate 20. Then, a layer of titanium metal 22 with a thickness of about 50-150 .ANG. is formed, for example, by using a sputtering method, over the surface of the silicon substrate 20. Then, a layer of cobalt metal 24 is formed, for example, by using a sputtering method, over the titanium layer 22. The sputtering method comprises, for example, DC magnetically controlled sputtering method, at an base pressure of about 2.times.10.sup.-7 torr.
Next, referring to FIG. 2B, a thermal annealing method is performed, for example, by using a rapid thermal process with nitrogen for about 30 sec, so that the cobalt layer 24 reacts with the silicon atoms on the surface of the substrate 20, forming a layer of cobalt silicide 26. Simultaneously, a layer of titanium-cobalt-silicon-oxygen mixture 28 is formed on the surface. Then, the unreacted and remaining cobalt metal are removed, for example, by using a wet etching method.
There is still a rough interface between the silicon substrate 20 and silicon cobalt layer 26 in this conventional method. Moreover, a large amount of silicon is consumed and voids are formed at the interface.